Inner Orbital Ionization

Older work using electron spectroscopy to study the strong field ionization of molecules showed directly the population of the three lowest lying states of N.J the X, A, and B states, corresponding to the ionization of the 3ag, the 17ru, and the 2au orbitals [14]. More indirect work using VUV fluorescence identified the ionization of the significantly deeper 2ag orbital [42]. 

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The paper is organized as follows we first discuss vibrational excitation through various mechanisms, including ionization, / -dependent depletion, and bond-softening. We then present evidence for electronic excitation and consider multiphoton excitation, inner orbital ionization, and excitation through recollisions. Several applications of these interactions are presented, followed by our conclusions. 

Individual X-ray photons, 127 Induced birefringence, 89 Inner shell ionization, 123, 124 Inner-orbital ionization, 15 InP, 52... 

Important aspects of the interaction of strong laser fields with molecules can be missed in standard TOF experiments, most notably the population of electronically excited states. However, by studying vibrational excitation, the frequency and dephasing of the vibrational motion can be used to identify the electronic state undergoing the vibrational motion. In some cases, this turns out to be a ground state, and in others, an excited state. Once we have identified an excited state, we are left with the question of how and why the state was populated by the strong field. In one example above (the Ij A state discussed in Sect. 1.3.3), the excited state is formed by the removal of an inner orbital electron, in this case a iru electron. This correlates with the measured angular dependence for the ionization to this state. 

The first ionization potential refers to the most weakly bound electron of the neutral molecule in the dilute gaseous phase, i.e. the energy liberated by removing an electron from the highest occupied orbital. Several experimental methods are available for measuring these for molecules and theoretical models can be set up so as to correlate them. Other ionization potentials may similarly be observed if, instead, an electron from an inner orbital is removed from the neutral molecule. Thus a molecule will have as many ionization potentials as occupied orbitals. 

The best estimates have been obtained to date by using the MINDO and PNDO methods. In Tables 6 to 8 we show the ionization potential values obtained by each of these methods for alkanes and cycloalkanes, alkenes, acetylenes and aromatic compounds. Dewar and Klopman (PNDO) and Dewar et al. (MINDO/2) also compared their calculated inner orbital energies with experimental ionization potentials obtained from photoionization spectra. The ionization potentials of methane and ethane have also been calculated by the PNDO method along the more sophisticated procedure of minimizing separately the energy of the ion and that of the molecule. In these cases, the experimental value of the first ionization potential was reproduced accurately 48>. 

Table 6.3 Values of the parameters sni and rjni for the inner shell ionization in the Is, 2s, 3s, 3p, and Id orbits...

Although originally mainly the more pronounced near-edge features were observed and compared or interpreted (usually empirically), it is evident from a recent review (18) that today considerably greater emphasis is given to the study of the extended fine structure. This stands in relation to the improved experimental methods for detection of weak modulations and to the currently more advanced theoretical description of the EXAFS part of the spectrum. Complete understanding of the Kossel structure at the threshold part of an element s inner-shell spectrum, which contains among others valence orbital, ionization, and chemical shift information, is relatively slow due to the... 

Two charge-transfer bands were observed for furan, thiophene, and tellurophene and only a single band for selenophene, because of the small difference in energy between the two upper molecular orbitals of this ring (see. Section II,B,7). The spectral data, the stability-constant value, and the empirical calculation of ionization energies are in favor of a n - 7T nature of these complexes and indicate that the inner orbitals of the donors are involved in the charge-transfer interaction. 

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